Weightlessness and the Normal Force

One of the most visually inspring aspects of space exploration is when we are able to witness astronauts interacting with the gravitational force different than that close Earth's surface. Humans have only witnessed outer space in places that have a low gravitational force, for example, the moon and the International Space Station. 


Spacewalk on the ISS
1h:29m:25s for weightlessnes spacewalk scene

This clip is of astronaut Andrew Morgan doing a spacewalk to make a repair on the ISS. Notice how his legs are floating behind him and he is seen at times clutching to the ship with a single hand to control his entire body. This is because Andrew and the ISS are undergoing a constant free fall directed to the center of Earth but are stuck in orbit around it. Free fall scenarios occur when the only force that is acting on an object is gravity. The difference between an apple falling out of a tree and an object experiencing free fall in orbit is that the object in orbit travels around the Earth so fast that the distance between the Earth and that object does not decrease. This is because the force of gravity on objects far away is relatively small and is constantly changing in direction as it is center seeking.



During both scenarios, however, we are watching objects falling towards the Earth. We just get to watch the orbiting object fall forever! A common misconception is calling the state of free fall as someone being "weightless". There are a few problems with this. Being weightless is solely a feeling of having nothing pushing upwards on you, while free fall is defined as the state of constant acceleration due to the force of gravity. The normal force is what provides the feeling of weight. The normal force is almost always equal in magnitude and opposite in direction to the component of the force of gravity that acts on an object that is perpendicular to the surface that the object is on.

Block Sliding Down Incline

The incline is stuck to the surface of the Earth. Notice how the arrow of the Normal Force is the same size as the component of gravity that is opposing it.

In this system, the object is solely accelerating in the positive x direction as the normal force counteracts the force of gravity in the positive y direction. If this surface was flat, the force of gravity would be solely in the y direction and the normal force to counteract this would need to be equal in magnitude to the force of gravity but opposite in direction. This is somewhat intuitive because if there was not a normal force that counteracts the force of gravity then the object would fall through the ground. This is an example of Newton's third law. 

Now we are ready to analyze a system in which a person can experience the feeling of weightlessness. Take for example, the first video where Andrew Morgan is performing a space walk. 


This is a basic force diagram of what Andrew Morgan experiences when in orbit on the ISS. Since the ISS is moving at the same speed as him and is also undergoing free fall, he never experiences a normal force that counteracts the force of gravity.

Using Newton's second law, we can prove this.

ΣF=ma

mg-FN=ma
Due to free fall, a = -9.8m/s^2
m(9.8m/s^2)-FN=m(-9.8m/s^2)
0N=FN






Any being that is undergoing free fall feels weightless because there is no normal force that is acting on them. Astronauts on the internationals space station are always undergoing free fall and this is the reason why they cannot stay there for long periods of time. Astronauts often return from work on the ISS with health issues. 

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